The invention relates to an assembly of blades to the disc of a gas turbine engine rotor and more particularly to such an assembly employing ceramic blades for high temperature inlet conditions to the turbine and assembled to a generally low temperature ferritic alloy disc.
It is well known that the efficiency of a gas turbine engine can be increased by increasing the inlet temperature of the motive fluid. However, it is also well known that the temperatures of the turbine parts must be maintained in a range wherein such parts do not lose their strength or become easily attacked by the corrosive nature of the motive fluid.
High density, hot pressed silicon nitride, silicon carbide and other ceramic materials have the ability to withstand relatively high temperatures without loss of strength or incurring corrosive deterioration. Because such material is rather brittle and susceptible to failure under tensile stress (and thereby sensitive to stress concentrating notches) its use for rotating blades subjected to high centrifugal and bending forces in large gas turbine engines has not met with much success. However, see U.S. Pat. No. 3,943,703 as an example of a small gas turbine engine with certain ceramic components including the rotating blades, to increase the permissible temperature of the operation cycle.
Thus, for the most part, the turbine inlet temperatures have been limited to the range dictated by the high temperature super metal alloys which generally maintain their strength up to approximately 1600.degree.-1700.degree. F, whereas with ceramic blades it would be possible to increase the inlet temperature to 2300.degree.-2500.degree. F with a significant increase in turbine efficiency.
Also, because the high temperature metal alloys are rather expensive it is common to use such metal for the blades only and use a lesser expensive ferritic or low alloy metal rotor and integral disc in the gas turbine and cool the disc to the temperature of 600.degree.-800.degree. F to maintain it within an acceptable temperature range.